Merging Old Emails between Thunderbird and Zimbra Collaboration

I get this error message when trying to setup new email providers to my zimbra desktop. I use Zimbra Collaboration but I also have an old Thunderbird account with all of my old contacts and emails... which I would like to access along with my new Zimbra Collaboration. I thought that Zimbra Desktop would be the solution, but I am having issues setting up the accounts! I know the exact server name, port and other needed information, yet I still get the message below. The words in green have been changed, but everything else is the same.

Merging old emails

I use the same email server for both Thunderbird and Zimbra Collaboration. I can drag a copy of old emails from a local folder in Thunderbird to the Inbox, which allows me to view these emails in Zimbra as well.. unfortunately there is a max limit to how much it will hold. Also, I can not figure out how to setup local folders for Zimbra.. is this possible? I think that would be a way to get around any limits as I don't have admin access. But if I was able to get access, where would I look to raise those limits?